Field of the Invention
The present invention relates to minimally invasive physiological monitoring systems. More particularly, the present invention relates to an implantable probe for monitoring one or more parameters in the esophagus, such as pH, in connection with the detection of gastroesophageal reflux disease.
Description of the Related Art
Gastroesophageal reflux is a condition in which gastric acid refluxes, or flows in the direction opposite to the normal flow, from the stomach into the esophagus. Frequent reflux episodes may result in a potentially severe problem known as gastroesophageal reflux disease (GERD). GERD is the most common cause of dyspepsia or heartburn. GERD affects approximately 75 million adults in the United States on at least an intermittent basis, and approximately 13 million adults on a daily basis. As a common cause of chest pain, GERD frequently mimics the symptoms of a myocardial infarction or severe angina pectoris, which are signs of severe coronary artery disease. Because their treatments and outcomes are different, distinguishing between GERD and coronary artery disease is of paramount diagnostic importance to the patient and physician.
The lower esophageal sphincter (LES), or valve, is composed of a smooth muscle ring located at the gastroesophageal junction, and it plays a key role in the pathogenesis of GERD. Factors that cause or contribute to GERD include the following: transient relaxation of the LES, delayed stomach emptying, and ineffective esophageal clearance. Another cause of GERD is decreased resting tone of the LES, which produces incompetence (incomplete closing) of the LES.
At rest, the LES maintains a high pressure, between 10 and 30 mm Hg above intragastric pressure. Upon deglutition (swallowing), the LES relaxes before the esophagus contracts, allowing food to pass through into the stomach. After food passes into the stomach, the LES contracts to prevent the stomach contents, including gastric acid, from refluxing into the esophagus. The mechanism of the LES contraction and relaxation is influenced by vagus nerve innervation and hormonal control by gastrin and possibly other gastrointestinal hormones.
Complications of GERD include esophageal erosion, esophageal ulcer, and esophageal stricture. Stricture formation results from scarring of the esophagus following prolonged exposure of the esophageal mucosa to acid reflux. The most common clinical manifestation of stricture is dysphagia (difficulty swallowing). Unlike dysphagia from nonstrictured esophageal reflux, dysphagia caused by stricture is a progressive disorder in that the size of a bolus which can pass into the stomach becomes progressively smaller. Prolonged exposure of esophageal mucosa to acid often leads to a precancerous condition known as Barrett's esophagus. Barrett's esophagus is characterized by the replacement of the normal squamous epithelium that lines the esophagus with abnormal columnar epithelium. Barrett's esophagus is clinically important not only as a marker of severe reflux, but also as a precursor to esophageal cancer.
Efforts have been made to define and report as reflux rapid changes of intraesophageal pH, even while the pH remains within the normal esophageal pH range of 4 to 7. Such pH changes, however, can be difficult to prove to be caused by true gastroesophageal reflux, and in some instances may not be caused by reflux.
Some have measured gastroesophageal reflux with radioisotope techniques. With these techniques, a radiolabeled meal is fed to the patient. With a gamma camera positioned externally on the patient's chest or internally within the esophagus, it is possible to detect gastroesophageal reflux containing the isotope, regardless of pH. The use of radioactive material and the expense of stationary or ambulatory gamma cameras make the radioisotope method for detection of reflux unattractive.
Intestinal impedance has previously been used as a surrogate for measurement of gastric emptying into the intestines. In such studies, a liquid or solid meal is administered to a patient, and changes in intestinal impedance are monitored from external electrodes around the abdomen.
The primary and most reliable method of objectively diagnosing GERD, however, is 24-hour measurement of pH within the lower esophagus. The normal pH range in the esophagus is between 4 and 7. As a general rule, when gastric acid enters the esophagus from the stomach, the intraesophageal pH drops below 4. An epoch of one second or more during which the intraesophageal pH falls below 4 is considered a reflux event.
Certain methods and apparatus are known in the prior art for 24-hour monitoring of intraesophageal pH in patients with suspected GERD. An example of a system for ambulatory 24-hour recording of gastroesophageal reflux is the Digitrapper™ System (manufactured by Synectics Medical AB, in Stockholm, Sweden) used with glass or Monocrystant™ pH catheters (as described in U.S. Pat. No. 4,119,498) and with the analysis software EsopHogram™ (by Gastrosoft, Inc. in Dallas, Tex.). These prior art systems typically measure pH in the esophageal tract with an intraesophageal catheter and generate reports regarding esophageal exposure of gastric juice.
Currently, ambulatory esophageal pH monitoring is performed by passing a pH catheter transnasally into the esophagus, to a point approximately 5 cm above the LES. The proximal end of the nasoesophageal catheter extends outside the patient's nose and is usually taped down to the cheek in two places and draped over the ear.
The use of this indwelling nasoesophageal catheter for ambulatory pH monitoring presents a number of disadvantages. Almost invariably, the catheter's presence is very uncomfortable to patients, who frequently develop a sore throat and rhinorrhea (runny nose) because of local irritation of oropharyngeal and nasopharyngeal mucous membranes, respectively, from the catheter. In addition, many patients are embarrassed to be seen in public with the catheter assembly attached to their faces. Furthermore, patients frequently experience an increased swallowing frequency when the catheter is in place, due to reflex stimulation. This increased swallowing introduces a significant amount of air into the stomach, which can cause abdominal discomfort. Finally, increased swallowing in response to the catheter's presence may erroneously raise a patient's intraesophageal pH readings because saliva is alkaline.
Thus, there remains a need for an ambulatory system that avoids the use of an indwelling nasoesophageal catheter during the assessment of esophageal pH and other physiological parameters to detect gastroesophageal reflux.